Experimental Investigation of Clean Spray Combustion under Varied Ambient Conditions.

摘要

In recent decades, environmental pollution becomes the one of the most serious global issues due to the pollution emissions from the fossil fuel combustion. Soot and NOx from diesel spray combustion make a large contribution in the pollution emissions from power and transportation sectors. With the stringent emission standards and the limited fossil fuel resources, clean spray combustion with a high efficiency needs to be developed in order to improve the environment and sustain the consumption of fossil fuel.;Experimental spray combustion investigation was conducted in an optically accessible constant-volume chamber using a single-nozzle fuel injector. The ambient O2 concentration was varied between five discrete values from 10% to 21% and three different ambient temperatures (800 K, 1000 K, and 1200 K). These conditions simulate different EGR levels and ambient temperatures in diesel engines. Three fuels (Ultra-low-sulfur Diesel, Jet-A, and biomass-based BTL) were used in this study.;First, multi-band emission measurement was conducted to illustrate the diesel and Jet- A flame development under different ambient conditions. The transient and quasi-steady state analyses were based on measurements using a high-speed video camera and an ICCD camera, respectively. Soot is seen to be oxidized more quickly for Jet-A than diesel at the end of combustion, evident by comparing the area of NL, especially under high O2 concentration. The quasi-steady state results suggest that soot is oxidized effectively under high O2 concentration conditions by the wider region of OH* in the downstream locations where only OH* emission is observed. Based on the imaging of multi-band flame emissions, conceptual flame structures were proposed to complement the previous conceptual models for spray combustion under different combustion modes.;Second, soot concentration (KL factor) and soot temperature were measured under different ambient conditions for a new biomass-based BTL, ultra-low sulfur diesel, and Jet- A. A high-speed camera was employed coupled with two bandpass filters (centered at 550 nm and 650 nm, both with 10 nm FWHM) to implement a two-color thermometry method and measure the soot concentration and temperature simultaneously. More soot is seen in the near-wall regions under the low-temperature combustion mode while high level soot is observed in the upstream and midstream for the conventional combustion mode. BTL combustion generates a lower integrated KL factor and soot temperature compared to diesel and jet-A fuels under all the experimental conditions. Finally, the characteristics of the two-color results were further discussed and analyzed.;Finally, the effects of different loads and the two-injection strategy were analyzed under three typical ambient conditions based on the two techniques mentioned. The results indicate that low load condition tends to have average lower soot with a similar average soot temperature compared to high load condition during the whole flame development except the case that a larger accumulation of soot occurs at the end of combustion under the high temperature and highly diluted combustion. The two-injection strategy with a pilot injection can be considered as an effective method to reduce soot and NOx simultaneously compared to single injections under the high temperature highly diluted combustion and the conventional combustion, characterizing a lower soot level, a smaller soot area and a short effective high soot temperature duration.